Microbubble elevator induced buoyancy oscillations of reacting droplets

Autonomous droplet oscillations are intriguing and provide a strong inspiration for the development of active soft matter. However, they are often limited by their reliance on external gradients and slow transport. Here, we demonstrate a propulsion mechanism driven by confined interfacial reactions: the hydrogen microbubble elevator. We show that hydrogen evolution within a liquid organic hydrogen carrier (LOHC) droplet and the resulting bubble pinch off modulate the droplet’s effective density, inducing periodic rising and sinking cycles in a stratified fluid. These droplets reach peak velocities of up to 25 mm/s over 2.5 cm trajectories - three orders of magnitude faster than typical Marangoni-driven bouncing - and sustain robust oscillatory motion for up to 25 minutes. The oscillation period and amplitude are programmable through the reaction kinetics, with motion persisting until the bubble-to-droplet size ratio exceeds a critical threshold. With a force balance model, we can quantitatively explain the experimental results and the interplay between gas evolution rates and hydrodynamic forces. Active soft matter systems, which convert free energy into mechanical work, face challenges in sustaining motion due to reliance on external gradients and slow transport mechanisms. This study introduces a propulsion mechanism using hydrogen microbubble elevators within reacting liquid organic hydrogen carrier droplets, achieving rapid and programmable buoyancy oscillations, significantly enhancing the distance and duration of autonomous droplet motion compared to traditional methods.